Children at Risk, the Houston-based advocacy group that issues annual ratings of Texas public schools, has had to correct the ratings it released late last month.

For the first time in the nine years of the rating system, the group hired an outside vendor to do some of the calculations and that vendor used an incorrect formula, said Caroline Neary, associate director for the group’s center for social measurement and evaluation.

Shortly after the rankings were released, several schools called to say that they had looked at their own data and come up with different results, she said. So all of the data was reanalyzed.

The results did not change the broad trends identified in the original rankings. Schools with a mostly low-income students tended to rank lower. Charter schools in North Texas were more likely to get lower rankings than noncharter schools. And they were also more likely to get lower rankings than charters elsewhere in the state.

The new calculations resorted the rankings, with most schools moving up or down a few positions. Most of the letter grades assigned to the schools did not change. For those that did, the change was small, the equivalent of an B- becoming a C+.

“As an organization, we are extremely humbled by this,” Neary said. “As soon as we saw there was an error, we immediately took everything down from our website.”

The organization also notified news organizations, including The Dallas Morning News. A corrected version of a chart that ran with The News’ April 27 report accompanies this story.

Neary said she hoped that her organization’s quick action will minimize the effect of issuing erroneous rankings on the group’s reputation.

“We can only apologize for any confusion or concerns this caused in the community,” she said.